G6iii / 17B10: Compare and contrast two methods of measuring cardiac output

17B10: Exam Report

Compare and contrast two methods of measuring cardiac output.

35% of candidates passed this question.

Good answers began with a definition of cardiac output. For each method, it was expected that candidates discuss the theoretical basis, equipment, advantages and disadvantages / sources of error and limitations. Additional marks were awarded when an attempt was made to compare and contrast the two methods (often helped by the use of a table).

G6iii / 17B10: Compare and contrast two methods of measuring cardiac output

Cardiac Output

  • CO = HR x SV = volume of blood ejected by heart per minute
  • Determinants of CO are HR and SV

Techniques to Measure CO

Inavasive

Non Invasive

PAC

  • Dye Thermodilution

Fick Principle

TTE

TOE

Oesophageal Doppler

Pulse Contour Cardiac Output

  • PICCO
  • LiDCO

Partial CO2 rebreathing

Dye Thermodilution

Transoesophageal Echocardiography

Theory

CO measurement is based on the FICK PRINCIPLE

“Amount of indicator substance taken up over time = A – V difference of the substance x blood flow” 

  • Current thermodilution practice of measuring CO uses a modified STEWART HAMILTON EQUATION
Flow
  • The marker is temperature

Uses Doppler technique to monitor cardiac function by multiplying the velocity-time integral (VTI) of blood flow through the transversal area of the vessel (CSA) with HR

 

CO = CSA * VTI * HR

 

Equipment

 

Thermistor-tipped Swan Ganz Catheter

Cold D5W

STEPS

  1. Bolus of sterile solution (injectate) colder than pt blood is injected into RA port
  2. Injectate mixes with RA blood → TV → RV → PA
  3. Thermistor located 4cm from tip of catheter situated in PA detects ∆blood temp as blood passes catheter tip
  4. Temp ∆ detected produces a curve which shows: ∆TEMP vs TIME calculated by a computer & converted into a measurement of CO by S – H application

Q = CO

V = volume of injectate

TB = temp blood

T1 = temp injectate

K = constant, which corrects for specific heat & density of inectate

dt = ∆ time

SH Equation
  • AuC is inversely related to CO
  • Small AuC = quick CO
  • Larger AuC = slow CO
  • Mean of 3 measurements is taken
  • Mean has to be 15% different to previous mean → otherwise there has been a margin of error
  • Transoesophageal ultrasound transducer
  • Skilled operator
  • Anaesthetised patient

The TOE probe is introduced into the mouth and passed down the oesophagus to visualise the Aortic Valve then
inserted to the gastric cavity to obtain transgastric view of LVOT

Aortic Blood flow is obtained by pulsate Doppler

The VTIAORTIC os calculated by outlining the Aortic blood flow

Advantages

 

  • Non-toxic indicator
  • Repeated measurements easily
  • Computer performs all patented algorithms

 

  • Direct measurements in real time
  • Cardiac anatomy

Disadvantages

 

Associated w increased M&M:

  • Arterial injury
  • PTx
  • Chylothorax
  • Nerve injury
  • PA rupture
  • Valve damage

 

  • Risk of oesophageal perforation
  • Odynopahgia
  • Harmorrage
  • ETT malpositioning
  • Dental injury

Error/Limitation

 

Factors compromising technique:

  • Shunts
  • TR
  • Cardiac Arrythmias
  • Abnormal respiratory patterns
  • Low CO state

Assumes:

  • Constant flow
  • Normal heart & valves
  • No resp. fluctuations

 

  • Requires advanced training & expertise

Comparison

  • PAC: constant PAP, frequent CO, sampling of SVO2 – however these values are modified by artificial ventilation and ventricular non compliance
  • TOE: gives direct measurements of values whereas PAC is indirect. Most PAC values can be calculated by TOE, except for SVO2
  • Safety: PAC has a higher complication rate (1-5% cf TOE 0.1-0.2%)